Luminescent composition

ABSTRACT

The invention relates to a luminescent composition which is based on yttrium oxide sulfide and other oxide sulfides and to which at least one doping agent is added. The inventive composition has a characteristic emission spectrum and can optionally be used along with a reading system that is adjusted to the emission spectrum in order to mark substances or *substance mixtures.

This application is a continuation of U.S. application Ser. No.12/306,567 filed Feb. 16, 2009, which is a 371 of InternationalApplication No. PCT/EP2007/005688, filed Jun. 27, 2007, both of whichare incorporated by reference.

The invention relates to a luminescent composition which is based onyttrium oxide sulfide and further oxide sulfides and to which at leastone dopant has been added. The composition has a characteristic emissionspectrum and can, if appropriate together with a reading system matchedto the emission spectrum, be used for marking materials or mixtures ofmaterials.

Compounds containing lanthanide ions in the oxidation state +3 are oftenluminophors which on excitation with radiation in the infrared rangeemit shorter wavelength light, e.g. in the visible range and/or in theUV range. This property, referred to as “up conversion” or “anti-Stokesfluorescence”, can be attributed to the electrons of the 4 f shell oflanthanide ions being raised by sequential multiple excitation onirradiation to an energy level which has been increased by more than theenergy corresponding to absorption of a single photon. A photon whichhas a higher energy than the originally absorbed photon can be emittedfrom this energy level on relaxation.

The use of lanthanide oxide sulfides as anti-Stokes luminophors isdescribed, for example, in WO 00/60527 and in the U.S. Pat. Nos.6,802,992 and 6,686,074. Furthermore, the use of these lanthanide oxidesulfides for marking materials or mixtures of materials is known.

According to the present invention, novel luminescent compositions whichare based on the oxide sulfides of yttrium and at least three otherelements and to which at least one dopant, preferably selected fromamong oxides and fluorides of main group and transition group elements,has been added are provided.

The present invention firstly provides a luminescent compositioncomprising

-   -   (a) an oxide sulfide of yttrium and oxide sulfides of at least        three further elements selected from among lanthanum, cerium,        praseodymium, neodymium, samarium, europium, gadolinium,        terbium, dysprosium, holmium, erbium, thulium, ytterbium and        lutetium and    -   (b) at least one dopant selected from among oxides and fluorides        of main group and transition group elements.

The composition of the invention is a luminophor having “up converter”and/or “anti-Stokes” properties. It is preferably in crystalline form.Furthermore, it is preferred that the composition consists of a singlephase, for example a crystalline phase, which can be established byX-ray diffraction methods. The composition is usually in the form ofparticles having an average particle size of ≧50 μm, in particular ≧1nm. The particle size is preferably in the range 1 nm-100 μm, preferably5 nm-50 μm and particularly preferably about 100 nm-10 μm.

Component (a) of the composition is formed by an oxide sulfide ofyttrium and oxide sulfides of at least three further elements asindicated above. Yttrium and the further elements are usually present astrivalent cations, so that component (a) of the composition of theinvention can preferably be represented as follows:

Y₂O_(2+x)S_(1−x)x(M¹/M²/M³)₂O_(2+x)S_(1−x)

where M₁, M₂ and M₃ are trivalent cations of at least three of theabovementioned elements and X is a number in the range from 0 to 0.5,preferably from 0 to 0.2. Particular preference is given to X being 0.

In the total component (a), the yttrium oxide sulfide is preferablypresent in a proportion of ≧90 mol %, particularly preferably ≧92 mol %,even more preferably ≧94 mol % and most preferably ≧96 mol %. Thefurther oxide sulfides are preferably present in a proportion of in eachcase up to 2 mol % based on the total component (a). The further oxidesulfides are preferably selected from among oxide sulfides of erbium,ytterbium and at least one further element, in particular lutetium,gadolinium, holmium, thulium, dysprosium and/or europium. The oxidesulfides of erbium and ytterbium are preferably present in a proportionof in each case 0.5-2 mol %, particularly preferably 1-2 mol %, based onthe total component (a). The further oxide sulfides are preferably usedin smaller proportions of, for example, 0.1-1 mol %, particularlypreferably 0.1-0.5 mol %, based on the total component (a).

For example, the component (a) of the composition can contain oxidesulfides of 3, 4, 5, 6, 7 or even more further elements in addition tothe oxide sulfide of yttrium.

The composition of the invention additionally contains, as component(b), at least one dopant selected from among oxides and fluorides ofmain group and transition group elements. The dopants are preferablypresent in a proportion of in each case up to 5 mol %, particularlypreferably in each case up to 2 mol %, even more preferably in each caseup to 1 mol %, even more preferably 0.05-1 mol % and most preferably0.1-0.2 mol %, based on the sum of the components (a) and (b).

A preferred dopant is a fluoride, which can be used, for example, as analkaline earth metal fluoride or as an alkali metal fluoride, e.g. aspotassium fluoride. The fluoride is preferably present in a proportionof 0.1-0.2 mol %, based on the sum of the components (a) and (b).

Further preferred dopants are alkaline earth metals and/or transitiongroup elements which are present as cations bearing two or even morepositive charges, preferably in the form of oxides and/or fluorides.Particularly preferred dopants are calcium, zinc and/or titanium, forexample in the form of the oxides calcium oxide, zinc oxide or titaniumdioxide. The cationic dopants are preferably present in a proportion ofin each case 0.1-0.2 mol %, based on the sum of the components (a) and(b).

The luminescent compositions comprising the components (a) and (b)firstly have a high luminescence intensity and secondly have emissionlines or peaks which are characteristic of the presence and proportionsof the individual components. Thus, specific combinations of oxidesulfides and dopants make it possible to obtain a virtually unlimitednumber of different emission spectra which can be detected by means of areading system matched specifically to the respective spectrum.

The compositions of the invention can be produced by homogenizingyttrium oxide (Y₂O₃) powder with oxides of the other elements, e.g.ytterbium oxide (Yb₂O₃), erbium oxide (Er₂O₃) and other oxides such asHo₂O₃, Lu₂O₃ and/or Gd₂O₃, and also the dopants or precursors thereof,e.g. TiO₂, CaCO₃, ZnO and/or KF, by milling and subsequently sinteringthe mixture at elevated temperature, e.g. 1200-1700° C., in a furnace,preferably in air, in order to achieve homogeneous distribution of thecations in the crystal lattice. The sintered product is subsequentlymilled and reacted with H₂S at temperatures in the range from 700° C. to1000° C., preferably for 2-12 h, giving a uniform phase based on Y₂O₂Sand containing further oxide sulfides and also the dopants. The additionof fluoride as dopant leads to a homogeneous distribution of thelanthanide ions in the host lattice during the sintering process. Theaddition of dopants, e.g. polyvalent cations and/or fluoride, bringsabout drastic changes in the position and/or intensities of individualemission wavelengths. Furthermore, a large increase in the totalluminescence intensity occurs. It is assumed that a three-photonabsorption takes place in addition to the two-photon absorption knownfor anti-Stokes materials.

The luminophore of the invention can be used as detection and markingmaterials, for example as safety marking of materials or mixtures ofmaterials. In this way, the authenticity of products or documents can bedetermined. The luminophor can, since it is chemically inert, beintroduced into any solid and/or liquid materials or mixtures ofmaterials or be applied thereto. For example, the luminophor can beapplied to or introduced into carrier substances such as surface coatingcompositions, toners, inks, paints, etc, or products such as plastics,metals, glass, silicones, paper, rubber, etc. The luminophor ispreferably added to the product or part of the product in an amount of10-50 ppm, preferably 50-200 ppm. The luminophor of the invention isalso suitable for use in biological systems, e.g. cell cultures, samplesof body fluids or tissue sections or as contrast enhancer. Here, theluminophor in nanoparticulate or microparticulate forms can be coupledto biological detection reagents. Furthermore, the surfaces of particlesof the luminophor can be modified by means of deodetomines or otherbonding substances in order to improve the suspending properties, e.g.in organic liquids such as oils, naphthas, liquefied gases, etc., inaqueous liquids such as body fluids, in aqueous-organic liquid systemsand flowable powders such as toners. The smaller the particles, thelower is the tendency for sedimentation to occur. The particle size can,for example, be reduced by intensive milling to such an extent, e.g. to≧100 pm, that a stable suspension of the particles in liquids isachieved even without the addition of bonding substances.

Security against falsification of the marking is provided by theemission lines characteristic of the respective luminophor representinga cryptographic key which can be detected by a detector, i.e. the lock,matched to the respective material.

The detection of the presence of the luminophor can be effected byirradiation with a wavelength in the infrared range, in particular withIR monocoherent laser light or with an IR light-emitting diode havingwavelengths in the range from about 850 to 1500 nm, preferably fromabout 920 to 1000 nm, particularly preferably about 950-1000 nm, mostpreferably from 920 to 985 nm, with the luminophor being excited and theemitted radiation in the range of wavelengths characteristic for therespective luminophor, for instance in the range from 300 to 1700 nm,being detected. Irradiation is preferably carried out at a power of1-200 mW, in particular 10-80 mW. The irradiation of the productcontaining the luminophor can be carried out directly or by means of anoptical waveguide or another optically relevant transfer medium, e.g. anoptical solid body, a fluid, gas, etc. Detection can be effectedvisually or by means of detectors.

It is possible to use, for example, optical waveguides whose heads areground as collecting lenses so that incident light (IR light) and lightemitted by the luminophor (specific emission spectrum) form one unit andcan be focused at the same point. An advantage is that no mechanicalmisalignment between receiver and transmitter can occur. The dampingfactor of the optical waveguide, e.g. of glass or plastic, can vary,with the transition from the optical components (radiation source ordetection element) to the optical waveguide being constructed so as tobe low in covision. The length of the optical waveguide can vary and istypically in the range from 1 cm to 50 cm.

In a particularly preferred embodiment, a luminophor having acharacteristic emission spectrum is detected by means of a readingsystem matched to this emission spectrum. The reading system comprises aradiation source, preferably a radiation source in the IR range, and oneor more optical detection elements which are provided for the selectivedetection of specific emission lines of the luminophor, e.g. in respectof the wavelength and/or intensity. The detection elements can be, forexample, diodes, photoelements or electronic detectors. Preference isgiven to using detector matrices having a plurality of preferablydifferently set detectors, e.g. diode matrices, photoelement matrices orCCD matrices. The detectors or individual detectors of the detectormatrix can be combined with optical filters, e.g. bandpass filters,which can also be vapor deposited on the detection element. The filtersare preferably selected so that they allow passage of light in only aparticular wavelength range, e.g. a range of 5-15 nm, preferably about10 nm. The filters preferably contain high- and low-refraction layerssuch as TiO₂ and SiO₂. This ensures that bandpass filters having verysmall rise-fall flanks per optical element are provided. The passage oflight which does not correspond to the wavelength characteristic of theluminophor is prevented.

The use of detectors or detector matrices which detect a plurality ofemission lines of differing wavelength, e.g. 2, 3, 4 or more emissionlines, which are characteristic of a particular luminophor makes itpossible to provide a verification system having a high degree ofsecurity. The reading system may, if appropriate, also contain detectorswhich operate at wavelengths at which there is no emission line and thusserve as negative control.

The reading system can also, if appropriate, contain a programmableelectronic unit which can be reprogrammed to other emission lines whenrequired.

Furthermore, a plurality of different luminophors which can be evaluatedeither visually on the basis of different colors and/or by means ofdetectors can be applied to a product or a carrier. These differentapplications can be arranged beneath, above or next to one another, sothat a complex and characteristic pattern is obtained. For example, whentwo different luminophors are applied next to one another on a product,irradiation with a suitable IR source results in emission of twodifferent colors, giving a flip-flop effect.

The verification system according to the invention can also be combinedwith other verification systems, e.g. systems based on bacteriorhodopsinor specific DNA sequences.

Furthermore, the present invention is illustrated by the followingexample.

EXAMPLE 1 Production of a Luminophor

Pulverulent yttrium oxide was milled together with pulverulent oxides ofytterbium and erbium, in each case in proportions of 1-2 mol %, andother lanthanide oxides such as oxides of holmium, lutetium and/orgadolinium, in each case in proportions of 0.1-0.5 mol %, and alsodopants TiO₂, CaCO₃, ZnO and/or KF, in each case in proportions of0.1-0.2 mol %, in a ball mill for 3 hours. The resulting mixtures weresintered in air at 1500° C. in a furnace for 24-72 hours. The phasepurity of the resulting sintered products was confirmed by X-raydiffraction. The sintered products were subsequently milled and theresulting powders were reacted with H₂S at temperatures in the rangefrom 800° C. to 900° C. for 2-12 hours. Phase-pure crystalline compoundsof the Y₂O₂S type were obtained, as was confirmed by X-ray diffraction.

FIG. 1 shows the X-ray powder diffraction measurement on a sample of thecomposition (Yb, Er, Lu, Y)₂O₃ in the ratio 1.0:1.0:0.5:97.5. Themeasurement was carried out using a Siemens D5000 diffractometer (copperK-alpha radiation). Only lattice reflections of the host lattice Y₂O₂Scould be seen (formation of mixed crystals by substitution). Only aslight shift in the reflections caused by other ions incorporated intothe host lattice can be observed. Since no further reflections arepresent, the material is a phase-pure crystalline product containing nofurther crystalline phases (secondary phases).

1. A luminescent composition comprising (a) an oxide sulfide of yttriumand oxide sulfides of at least three further elements selected fromamong lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium andlutetium and (b) at least one dopant selected from among oxides andfluorides of main group and transition group elements, wherein thefurther oxide sulfides are selected from among oxide sulfides of erbium,ytterbium and of at least one further element, in particular oflutetium, gadolinium, holmium, thulium, dysprosium and/or europium. 2.The composition as claimed in claim 1, characterized in that yttriumoxide sulfide is present in a proportion of ≧90 mol % based on the totalcomponent (a).
 3. The composition as claimed in claim 1, characterizedin that the further oxide sulfides are present in a proportion of ineach case up to 2 mol % based on the total component (a).
 4. Thecomposition as claimed in claim 1, characterized in that it contains afluoride as dopant.
 5. The composition as claimed in claim 1,characterized in that it contains an alkaline earth metal and/or atransition group element as dopant.
 6. The composition as claimed inclaim 5, characterized in that it contains calcium, zinc and/ortitanium.
 7. The composition as claimed in claim 1, characterized inthat the dopants are present in a proportion of in each case up to 5 mol% based on the sum of the components (a) and (b).
 7. The composition asclaimed in claim 8, characterized in that the dopants are present in aproportion of in each case up to 1 mol % based on the sum of thecomponents (a) and (b).
 9. The composition as claimed in claim 1,characterized in that it is in crystalline form.
 10. The composition asclaimed in claim 1, characterized in that it is present as a singlephase.
 11. The composition as claimed in claim 1 in the form ofparticles.
 12. The composition as claimed in claim 11, characterized inthat the average particle size is in the range 1 nm-100 μm.
 13. Thecomposition as claimed in claim 1 introduced into or applied to amaterial or a mixture of materials.
 14. The composition as claimed inclaim 13 characterized in that it is present in a proportion of 10-500ppm in the material or mixture of materials.
 15. The use of aluminescent composition as claimed in claim 1 for marking materials ormixtures of materials.
 16. The use as claimed in claim 15, characterizedin that a plurality of different compositions are introduced as mixtureor as pattern into the material or the mixture of materials or appliedthereto.
 17. The use as claimed in claim 11, characterized in that aluminescent composition having a characteristic emission spectrum isdetected by means of a reading system matched thereto.
 18. The use asclaimed in claim 11, characterized in that the composition is excited toluminescence by irradiation with a wavelength in the range of about850-1500 nm, in particular about 920-1000 nm, and the emitted radiationin a range of 300-1700 nm is detected.
 19. The use of a reading systemfor detecting a marked material or mixture of materials, comprising: (i)a radiation source, preferably a radiation source in the IR range, and(ii) one or more optical detectors provided for the selective detectionof specific emission lines of a luminophor, wherein the material or themixture of materials is marked with at least one luminescent compositionas claimed claim
 1. 20. The use as claimed in claim 19, characterized inthat a plurality of optical detectors are provided for the selectivedetection of different emission lines.
 21. A material or mixture ofmaterials into which a composition as claimed in claim 1 has beenintroduced or applied thereto.